1. Field of the Invention
The invention relates to position sensing apparatus and more particularly, to acoustic transducers for use in acoustic position sensing apparatus.
2. Description of the Prior Art
Position sensors for producing an electronic signal representing the position of an object are essential parts of many complex electromechanical devices. Examples of the use of such position sensors abound, in everything from automobiles to photographic cameras. In many such applications, non-contact position sensing is desired, where mechanical contact could damage the object, such as a photographic film in a camera, or impede a delicate movement, such as the movement of a shutter blade in a photographic camera.
One type of non-contact position sensor shown in U.S. Pat. No. 3,694,800 issued Sept. 26, 1972 to L. F. Frank, employs the physical principle that the resonant frequency of an acoustic resonator having a port changes as a function of the obstruction of the port. The position sensing apparatus includes a "Y" shaped tube for defining an acoustic resonator. A first acoustic signal generating transducer (e.g. a loud speaker) is connected to one branch of the "Y" shaped tube for generating an acoustic signal of predetermined frequency in the acoustic resonator. The stem of the "Y" shaped tube is open, to define a sensing port. The resonant frequency of the acoustic resonator varies as a function of the position of an object near the sensing port. An acoustic signal sensing transducer (e.g. a microphone) is connected to the other branch of the "Y" shaped tube, and produces a signal representing the resonant condition in the acoustic resonator. Depending upon the size of the resonator, the size of the sensing port, and the frequency of the acoustic signal generator, the signal produced by the second acoustic transducer will be a maximum when the sensing port is completely unobstructed (open tube resonance), or completely covered (closed tube resonance). Signal processing electronics responsive to the signal produced by the acoustic sensing transducer produces a signal representative of the position of an object with respect to the sensing port. In the acoustic position sensing apparatus shown in the aforementioned U.S. patent, the acoustic signal transducers comprise acoustic transducers of the type used in hearing aids.
A problem encountered in employing such prior art acoustic position sensors in small mass produced apparatus such as photographic cameras is the difficulty in mass producing the acoustic transducers and incorporating the transducers in small areas, due to their physical size and complexity. Although the signal processing electronics are readily miniaturized and mass produced using well known electronic miniaturization techniques, it would not be readily apparent how the acoustic transducers could be simplified to be easily mass produced and miniaturized.
This problem becomes particularly acute when one attempts to employ a number of acoustic position sensors in one apparatus.
The object of the invention is to provide acoustic transducers for acoustic position sensing apparatus that are simply constructed and that have improved mass productivity, and are easily miniaturized for employment in compact mass produced apparatus.